Influence of Au-Based Metallization on the Phase Velocity of GaN on Si Surface Acoustic Wave Resonators

This letter presents a comparison between the phase velocity of Al and Ti/Au metallized GaN/Si-based surface acoustic wave structures. The resonance frequency (and consequently the phase velocity) is higher for Ti/Au-based metallization, especially when the acoustic wavelength has comparable values with the GaN layer thickness. Simulation results are in excellent agreement with the experiment. For example, the phase velocity on a 1-μm -thin GaN layer with 200-nm finger/interdigit spacing width of the interdigitated transducer (IDT) is 3707 m/s (simulated value: 3840 m/s) for 100-nm-thin Al metallization and 4360 m/s (simulated value: 4240 m/s) for Ti/Au 5-/95-nm-thin metallization. This behavior was explained considering the effects of the significantly higher value of the acoustic impedance of Au compared with Al. The simulated mode shapes have confirmed this explanation. The advantage of the Ti/Au metallization lies in the possibility to obtain a higher resonance frequency for the same topology of the IDT.